Extreme Programming
Kiran Pamnany
Software Engineering
 Computer programming as an engineering
profession rather than an art or a craft
 Meet expectations:
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Functionality
Reliability
Cost
Delivery schedule
Methodologies
Methodology: codified set of
recommended practices
No consensus:
 Waterfall model
 Spiral model
 Rational Unified Process (RUP)
 Extreme Programming (XP)
Classic process steps
 Requirements Analysis
 Specification
 Design and Architecture
 Coding
 Testing
 Documentation
 Maintenance
Waterfall model
 Proposed in 1970 by W. W. Royce
 Development flows through steps:
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Requirements analysis
Architectural design
Detailed design
Coding, debugging and unit testing
Integration and system testing
Deployment, operation and maintenance
Waterfall model (cont.)
 Pros:
 Track progress easily due to clear stages
 Easily identifiable milestones and deliverables
 Cons:
 Inflexible: difficult to respond to changing
requirements
 Design and coding discover requirements
inconsistencies
 Some problems not discovered until system testing
Spiral model
 Defined in 1986 by Barry Boehm
 Modified waterfall
 Software is developed in a series of
incremental releases
 Early releases are prototypes
 Later releases become increasingly complete
 Receive feedback after each release
Spiral model (cont.)
 Pros:
 Systematic and stepwise, but in an iterative
framework
 Estimates get more realistic as work progresses
 Some ability to cope with changing requirements
 Cons:
 Time-intensive process
 Not extensively used
Rational Unified Process (RUP)
 Defined in 1997 by Grady Booch, Ivar
Jacobson and James Rumbaugh
 General framework to describe specific
development processes
 Designed to be tailored for a given software
project with consideration for its size and type
 Recognized to be particularly applicable to
large projects with large teams
RUP Phases
 Inception
 Shared understanding of the system with the
customer
 Elaboration
 Architecture to build the system
 Construction
 Developing the system
 Transition
 Customer takes ownership of system
RUP Guidelines
 Develop iteratively
 Deal with changing requirements
 Address high risk items as the highest priority
tasks at each iteration
 Ideally, each iteration has an executable release
 Manage requirements
 Document functionality, constraints, design
decisions, business requirements
 Define use cases and scenarios
RUP Guidelines (cont.)
 Use component architecture
 For extensibility and reusability (CORBA/COM)
 Model software visually
 Abstraction using UML
 Verify software quality
 Plan quality control and assessment
 Involve all team members
 Control changes to software
 Use secure workspaces
RUP Workflows - Typical Project
(Source: George Stepanek, 2004)
RUP Criticism
‘High ceremony methodology’
Bureaucratic: process for everything
Slow: must follow process to comply
Excessive overhead: rationale,
justification, documentation, reporting,
meetings, permission
Extreme Programming (XP)
Formulated in 1999 by Kent Beck, Ward
Cunningham and Ron Jeffries
Agile software development
methodology (others: Scrum, DSDM)
Developed in reaction to high ceremony
methodologies
XP: Why?
 Previously:
 Get all the requirements before starting design
 Freeze the requirements before starting
development
 Resist changes: they will lengthen schedule
 Build a change control process to ensure that
proposed changes are looked at carefully and no
change is made without intense scrutiny
 Deliver a product that is obsolete on release
XP: Embrace Change
 Recognize that:
 All requirements will not be known at the beginning
 Requirements will change
 Use tools to accommodate change as a
natural process
 Do the simplest thing that could possibly work
and refactor mercilessly
 Emphasize values and principles rather than
process
XP Practices
(Source: http://www.xprogramming.com/xpmag/whatisxp.htm)
XP Practices: Whole Team
 All contributors to an XP project are one team
 Must include a business representative--the
‘Customer’
 Provides requirements
 Sets priorities
 Steers project
 Team members are programmers, testers,
analysts, coach, manager
 Best XP teams have no specialists
XP Practices: Planning Game
 Two key questions in software development:
 Predict what will be accomplished by the due date
 Determine what to do next
 Need is to steer the project
 Exact prediction (which is difficult) is not
necessary
XP Practices: Planning Game
 XP Release Planning
 Customer presents required features
 Programmers estimate difficulty
 Imprecise but revised regularly
 XP Iteration Planning
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Two week iterations
Customer presents features required
Programmers break features down into tasks
Team members sign up for tasks
Running software at end of each iteration
XP Practices: Customer Tests
 The Customer defines one or more automated
acceptance tests for a feature
 Team builds these tests to verify that a feature
is implemented correctly
 Once the test runs, the team ensures that it
keeps running correctly thereafter
 System always improves, never backslides
XP Practices: Small Releases
 Team releases running, tested software every
iteration
 Releases are small and functional
 The Customer can evaluate or in turn, release
to end users, and provide feedback
 Important thing is that the software is visible
and given to the Customer at the end of every
iteration
XP Practices: Simple Design
 Build software to a simple design
 Through programmer testing and design
improvement, keep the software simple and
the design suited to current functionality
 Not a one-time thing nor an up-front thing
 Design steps in release planning and iteration
planning
 Teams design and revise design through
refactoring, through the course of the project
XP Practices: Pair Programming
 All production software is built by two
programmers, sitting side by side, at the same
machine
 All production code is therefore reviewed by at
least one other programmer
 Research into pair programming shows that
pairing produces better code in the same time
as programmers working singly
 Pairing also communicates knowledge
throughout the team
XP Practices: Test-Driven
Development
 Teams practice TDD by working in short
cycles of adding a test, and then making it
work
 Easy to produce code with 100 percent test
coverage
 These programmer tests or unit tests are all
collected together
 Each time a pair releases code to the
repository, every test must run correctly
XP Practices: Design
Improvement
 Continuous design improvement process
called ‘refactoring’:
 Removal of duplication
 Increase cohesion
 Reduce coupling
 Refactoring is supported by comprehensive
testing--customer tests and programmer tests
XP Practices: Continuous
Integration
Teams keep the system fully integrated
at all times
Daily, or multiple times a day builds
Avoid ‘integration hell’
Avoid code freezes
XP Practices: Collective Code
Ownership
 Any pair of programmers can improve any
code at any time
 No ‘secure workspaces’
 All code gets the benefit of many people’s
attention
 Avoid duplication
 Programmer tests catch mistakes
 Pair with expert when working on unfamiliar
code
XP Practices: Coding Standard
Use common coding standard
All code in the system must look as
though written by an individual
Code must look familiar, to support
collective code ownership
XP Practices: Metaphor
 XP Teams develop a common vision of the
system
 With or without imagery, define common
system of names
 Ensure everyone understands how the system
works, where to look for functionality, or where
to add functionality
XP Practices: Sustainable Pace
Team will produce high quality product
when not overly exerted
Avoid overtime, maintain 40 hour weeks
‘Death march’ projects are unproductive
and do not produce quality software
Work at a pace that can be sustained
indefinitely
XP Values
Communication
Simplicity
Feedback
Courage
XP Values: Communication
 Poor communication in software teams is one
of the root causes of failure of a project
 Stress on good communication between all
stakeholders--customers, team members,
project managers
 Customer representative always on site
 Paired programming
XP Values: Simplicity
 ‘Do the Simplest Thing That Could Possibly
Work’
 Implement a new capability in the simplest
possible way
 Refactor the system to be the simplest possible
code with the current feature set
 ‘You Aren’t Going to Need It’
 Never implement a feature you don’t need now
XP Values: Feedback
Always a running system that delivers
information about itself in a reliable way
The system and the code provides
feedback on the state of development
Catalyst for change and an indicator of
progress
XP Values: Courage
Projects are people-centric
Ingenuity of people and not any process
that causes a project to succeed
XP Criticism
 Unrealistic--programmer centric, not business
focused
 Detailed specifications are not written
 Design after testing
 Constant refactoring
 Customer availability
 12 practices are too interdependent
XP Thoughts
 The best design is the code.
 Testing is good. Write tests before code. Code
is complete when it passes tests.
 Simple code is better. Write only code that is
needed. Reduce complexity and duplication.
 Keep code simple. Refactor.
 Keep iterations short. Constant feedback.
Software Quality: Another View
 A programmer presenting an elegant but
‘inefficient’ solution, talks of the inelegant but
‘efficient’ solution
 […] but your solution doesn’t work: if the
solution doesn’t have to work, then Begin..End
is a valid solution. Gerald Weinberg, ‘The
Psychology of Computer Programming’, 1972
Software Quality: Another View
 […] software engineering has accepted as its
charter “How to program if you cannot.” E.W.
Dijkstra, ‘The Cruelty of Really Teaching
Computer Science’, 1988
 Computer programming as a branch of
mathematics--formal provability of a program
is a major criterion for correctness
 Program correctness is ‘constitutional’; an
incorrect program is worthless or of negative
worth
Formal Verification
 The act of proving or disproving a system’s
correctness with respect to a formal specification
or property, using formal methods
 System types include FSM, Petri nets, timed and
hybrid automata, cryptographic protocols,
combinatorial circuits, etc.
 Properties to be verified are described in temporal
logics; approaches include state space
enumeration, abstract interpretation, etc.
Formal Methods
 Mathematical techniques for the specification,
development and verification of software and
hardware systems
 Classified as:
 Denotational semantics
 Operational semantics
 Axiomatic semantics
The Way Forward?
 ‘High ceremony’ software engineering
methodologies in disfavor
 Agile software development methodologies in
increasing use, but with significant criticism
 Formal methods will never have a significant
impact until they can be used by people that
don’t understand them. T. Melham